STM32F303 HAL made working; added SPI 3-way / half-duplex / bidi mode

[marnix]

See the commit texts for details.

Things I'd like to look at in the future:

• Splitting up the one big file into separate files.
• Looking at the other STM32 HALs, to make things consistent and extract common code.

Please note that I've tested this only with one piece of UART/SPI/I2C code (https://github.com/marnix/zig-stm32f3discovery-play/pull/31). I've tested this with Zig 0.11.0 and currently-latest ZigEmbeddedGroup/microzig 2873e9e0 + https://github.com/ZigEmbeddedGroup/microzig/pull/153 + https://github.com/ZigEmbeddedGroup/microzig/pull/152, and I didn't find any issues...

[marnix]

I've set this to 'draft' again, until https://github.com/ZigEmbeddedGroup/stmicro-stm32/pull/31 has been merged to main.